1. Field of the Invention
The present invention relates to an apparatus and a method for recording information on a recording medium such as an optical disk by applying a laser beam to an optical recording medium.
2. Description of Prior Art
An optical information recording medium such as an optical disk has been recently developed and marketed as an external memory of a computer and a medium for recording images and sounds. Optical disks that are already being practically used and on which data can be recorded include organic-dye-based, phase-change-type, and magneto-optic disks in which data is recorded by applying a laser beam to a recording film.
Pulse width modulation (hereinafter referred to as PWM) is known for recording data on an optical disk at a high density. The PWM technology performs modulation so that edges at the leading and trailing ends of record marks correspond to one of a digital signal. Such a technique is favorable in order to record data at a high density because more bits can be assigned in a record mark of the same length than the pulse position modulation technology for performing modulation so that the position of a record mark corresponds to one of a digital signal.
In the PWM technique, it is necessary to form a record mark without distortion; that is, it is necessary form the leading and trailing ends of the record mark in the same quality because the width of a record mark has information. Especially, when a long mark is formed in a phase-change-type optical disk, the width in the radial direction of a record mark increases toward the latter-half portion of the record mark due to the thermal storage effect of a recording film; that is, the record mark is distorted like a tear drop. In order to solve this problem, a recording method has been proposed for forming one record mark by applying a plurality of short pulse strings (e.g. Japanese patent laid-open Publication No. 185628/1991).
A general optical disk has a spiral track or concentric information tracks on the disk surface, and it records or reproduces information by applying a laser beam along the information track. In order to manage recording data easily, the information track is divided into sectors and addresses are given to the sectors.
The data format of an optical disk on which data can be recorded can be roughly classified into two types. One of the types has a sector format in which address sections and data sections are separated from each other. Magneto-optic disk and DVD-RAM standardized by ISO/IEC 10089 and the like belong to this type. Because the address sections are completely separated, address reproduction and data recording can be performed by time sharing without interfering each other even upon recording. Moreover, a buffer area, which is referred to as a gap area, is generally provided at a boundary between a address section and a data section. Therefore, by using the gap area, a recording apparatus can perform laser power control in any sector even when data is currently being recorded.
The other type has a sector format in which address sections and data sections are not separated from each other. CD-R, CR-RW, DVD-R, DVD-RW and the like belong to this type. Because there is no separated address section, it is possible to continuously record data, and it is possible to increase the recording density (format efficiency). However, it is necessary to reproduce addresses while recording data. Thus, it is necessary to control laser power under data recording.
A recording apparatus for CD-R controls optimizing a recording laser power with a process which is referred to as optimum power control (OPC) when information is recorded. OPC is performed by recording and reproducing predetermined data in a power calibration area (hereafter referred to as PCA) on an optical disk. For example, a test recording is performed to the PCA by changing the laser power in the unit of a sync frame and by reproducing the area to which the test recording is performed. Then, the laser power with which the best recording state is obtained is selected, and the selected laser power is used as the recording power for recording user data (refer to Orange Book of optical-disk recordable standards).
Moreover, there is another method which is referred to as running optimum power control (R-OPC) for controlling the recording power in accordance with the quantity of light that is reflected from an optical disk when user data is recorded. In R-OPC, the light quantity that is obtained by forming recording marks to record user data in the above-mentioned OPC is compared with the light quality that is obtained in the OPC, and the laser power is corrected in real time in accordance with the comparison result. As a mark part for obtaining the light quantity, an 11T mark is used (e.g. Japanese Patent laid open Publication No. 40548/1998). An 11T mark has a mark length which is equivalent to the longest mark length of a Eight-to-Fourteen-Modulation (EFM) code which is used for the CD standards, and the synchronization code of a sync frame has a 11T mark and a 11T space. Therefore, it has a mark length which appears at least once in a sync frame.
As mentioned above, in the continuous recording format such as CD-R/RW or DVD-R/RW, the light quantity of light that is reflected from a disk is changed according to the modulation of a laser beam upon data recording. Therefore, it is difficult to stably reproduce an address signal. Moreover, because there is not a period for controlling the laser power while recording user data, it is necessary to control the laser power in accordance with the light quantity of light which is emitted by a laser or the light quantity of light which is reflected from a disk while recording data. For example, in order to sample-and-hold a high power part of a laser beam during recording, it is necessary to generate a sampling pulse corresponding to the timing of the record pulse signal for modulating the laser.
However, in correspondence to recent applications of consumer appliances from audio recording to image recording, or in order to meet recent demands for the high speed operation speeds of peripheral units of a computer, the recording rate of an optical disk recording apparatus becomes higher. As the recording rate is increased, it becomes impossible to ignore influences on the stable sampling of a detection signal of transmission characteristics (propagation delay and settling) in a detection system for detecting the light quantity of an emitted laser beam or of a reflected light, fluctuations in power-supply voltage and temperature of the system and the like.
This is because when the recording rate is enhanced, the absolute time for the same mark length is shortened and, as a result, it is not easy to secure the time that is required to surely sample-and-hold the laser power. Moreover, the time axis of a monitoring signal for the light quantity of a emitted laser beam or a reflected laser beam to be sampled-and-held depends on the transmission characteristics (propagation delay and settling) in a detection system or the fluctuations in the power-supply voltage and temperature of the system. Therefore, a problem occurs in that the time margin for stable sampling-and-holding runs becomes short.
Moreover, as mentioned above, in the case of CD-R/RW, 11T, which is the longest mark length, appears at least once in a sync frame. The sync code of DVD-R/RW includes 14T only once, and 14T becomes a mark or a space depending on the series of modulated data just before the 14T mark. Therefore, a 14T mark does not always appear once in a sync mark. Further, the longest mark length of a 8-16 modulation code that is used for DVD-R/RW is equal to 11T, similar to the case of the EFM modulation code. However, the appearance probability of the 8-16 modulation code is very low, and it is not assured that the code appears at equal intervals. Therefore, particularly in the case of DVD-R/RW, there are problems in that the DVD-R/RW has a recording rate which is higher than the recording rate of CD-R/RW, and that it is difficult to obtain a mark length (or space length) which is capable of stably performing sampling-and-holding at a necessary time interval.